Begin by preparing the dressing: in a small bowl, whisk together the lime juice, fish sauce, rice vinegar, brown sugar, finely chopped garlic, and sliced red chilli with about one tablespoon of water. Stir well until the sugar completely dissolves, ensuring a perfectly balanced tangy-sweet marinade. Next, cook the brown rice noodles according to the package instructions. Once cooked, drain and immediately plunge them into a bowl of cold water to halt the cooking process and keep them pleasantly firm. Drain well and transfer into a large mixing bowl. Add thinly julienned carrots, finely shredded Chinese cabbage, and the delicate slices of spring onion. Pour over the dressing and toss gently but thoroughly, allowing every strand and vegetable piece to soak up the vibrant flavors. Heat the rapeseed oil in a frying pan over high heat until shimmering. Season the fillet steak well with salt and freshly ground pepper. Sear the steak in the hot pan for about 2 to 3 minutes on each side, aiming for a medium-rare finish that locks in the juices. Remove from heat and let the steak rest on a plate for 5 minutes; this resting period lets the juices redistribute and ensures tender, flavorful meat. Slice the rested steak thinly against the grain to maximize tenderness. To serve, divide the tossed noodle salad between two bowls, arrange the steak slices neatly atop, and garnish generously with fresh coriander leaves for an herby punch. For an extra hint of freshness, you might add a squeeze of lime or sprinkle additional sliced red chillies according to taste.